1. Field of the Invention
The present invention relates to a display device, and more particularly, to an organic electroluminescent device and a method of fabricating the same.
2. Discussion of the Related Art
In general, organic electroluminescent (EL) devices emit light by injecting electrons from a cathode and holes from an anode into a luminescent layer, combining the electrons and the holes to generate an exciton, and transitioning the exciton from an excited state to a ground state. Unlike liquid crystal display (LCD) devices, an additional light source is not necessary in the organic EL devices because the transition of the exciton between states causes light to be emitted from the luminescent layer. Accordingly, size and weight of the organic EL device is smaller than an LCD device. Since organic EL devices have lower power consumption, superior brightness, and fast response time, organic EL devices are being incorporated into consumer electronic products, such as cellular phones, car navigation system (CNS), personal digital assistants (PDA), camcorders, and palmtop computers. Moreover, since fabrication of organic EL devices is simple, it is much cheaper to produce organic EL devices than LCD devices.
Organic EL devices can be categorized into passive matrix organic EL devices and active matrix organic EL devices. Although the passive matrix organic EL devices have a simple structure and are formed using simple fabricating processes, the passive matrix organic EL devices require a relatively large amount of power to operate. Further, display sizes of the passive matrix organic EL devices are limited by their wiring structures. For example, as the total number of conductive lines increases, aperture ratios of the passive matrix organic EL devices decrease. In contrast, the active matrix organic EL devices have high luminescent efficiency and can produce high-quality images on large displays using relatively little power.
FIG. 1 is a cross-sectional view of an organic electroluminescent device according to the related art. As shown in FIG. 1, first and second substrates 10 and 60 face each other and are spaced apart from each other. An array layer “AL” is formed on the first substrate 10. The array layer “AL” includes a thin film transistor (TFT) “T” in each pixel region “P.” An organic electroluminescent (EL) diode “E” is formed on the array layer “AL.” The organic electroluminescent (EL) diode “E” includes a first electrode 48, an organic luminescent layer 54 and a second electrode 56. Light emitted from the organic luminescent layer 54 passes through one of the first and second electrodes 48 and 56 that is transparent.
Organic EL devices can be classified into a top emission type and a bottom emission type according to the emission direction of the light. For example, when the first electrode 48 is formed to be transparent so that light is emitted from a bottom of the organic EL device, the organic EL device is referred to as a bottom emission type. In a bottom emission type, the second substrate 60 is used as an encapsulation plate. Further, the second substrate 60 has a concave portion 62 and a moisture absorbent material 64 formed in the concave portion 62. The moisture absorbent material 62 eliminates any moisture and oxygen that may penetrate into the bottom emission type organic EL diode “E.” The first and second substrates 10 and 60 are attached with a seal pattern 70 at their periphery.
FIG. 2A is a plan view showing a pixel region of an organic electroluminescent device according to the related art and FIG. 2B is a cross-sectional view taken along a line “IIb-IIb” of FIG. 2A. In FIGS. 2A and 2B, a gate line 22 crosses a data line 42 to define a pixel region “P.” A buffer layer 12 is formed on a first substrate 10. A driving semiconductor layer 14 and a capacitor electrode 16 spaced apart from each other are formed on the buffer layer 12. A gate insulating layer 18 and a driving gate electrode 20 are sequentially formed on the semiconductor layer 14. The driving semiconductor layer 14 includes an active region “IIc” corresponding to the driving gate electrode 20 and source and drain regions “IIe” and “IId” at both sides of the active region “IIc.” A first passivation layer 24 is formed on the driving gate electrode 20 and the capacitor electrode 16 and a power electrode 26 is formed on the first passivation layer 24 over the capacitor electrode 16. The power electrode 26 is a portion of a power line 28 crossing the gate line 22.
A second passivation layer 30 is formed on the power electrode 26. The first and second passivation layers 24 and 30 have a first contact hole 32 exposing the drain region “IId” and a second contact hole 34 exposing the source region “IIe.” Further, the second passivation layer 30 has a third contact hole 36 exposing the power electrode 26. A driving source electrode 38 and a driving drain electrode 40 are formed on the second passivation layer 30. The driving source electrode 38 is connected to the source region “IIe” through the second contact hole 34 and the power electrode 26 through the third contact hole 36, while the driving drain electrode 40 is connected to the drain region “IId” through the first contact hole 32.
A third passivation layer 44 is formed on the driving source and drain electrodes 38 and 40. The third passivation layer 44 includes a drain contact hole 46 exposing the driving drain electrode 40. A first electrode 48 connected to the driving drain electrode 40 through the drain contact hole 46 is formed on the third passivation layer 44 in an emission area “EA” of the pixel region “P.” An interlayer insulating layer 50 is formed on the first electrode 48. The interlayer insulating layer 50 has an opening exposing the first electrode 48. An organic luminescent layer 54 is formed on the interlayer insulating layer 50 in the emission area “EA” and a second electrode 56 is formed on the organic luminescent layer 54. The organic luminescent layer 54 contacts the first electrode 48 through the opening of the interlayer insulating layer 50.
The driving semiconductor layer 14, the driving gate electrode 20, the driving source electrode 38 and the driving drain electrode 40 constitute a driving thin film transistor (TFT) “TD.” The organic EL device can have a switching TFT “TS” connected to the gate line 22 and the data line 42 and a driving TFT “TD” connected to the switching TFT “TS” and the power line 28. The driving gate electrode 20 is connected to the switching TFT “TS” and the drain electrode 40 has an island shape. The first electrode 48, the second electrode 56 and the organic luminescent layer 54 between the first and second electrodes 48 and 56 constitute an organic EL diode. Moreover, the capacitor electrode 16, the power electrode 26 and the first passivation layer 24 between the capacitor electrode 16 and the power electrode 26 constitute a storage capacitor “CST.”
In an organic EL device according to the related art, an array unit and an organic EL diode are formed on a first substrate, and a second substrate is attached to the first substrate for encapsulation. However, when the array unit and the organic EL diode are formed on one substrate in this way, production yield of the organic EL device is determined by multiplying the TFT's yield and the organic EL diode's yield. Since the organic EL diode's yield is relatively low, the production yield of the overall EL device is limited by the organic EL diode's yield. For example, even when TFTs are well fabricated, an organic EL device using a thin film of about 1000 Å thickness can be judged to be bad due to the defects in an organic EL layer. This results not only in the loss of the organic EL device but also in a loss of the TFTs that have to be discarded. Such loss wastes materials and increases production costs.
Bottom emission type organic EL devices have the advantages of high encapsulation stability and high process flexibility. However, the bottom emission type organic EL devices are ineffective as high resolution devices because they have poor aperture ratios. In contrast, top emission organic EL devices have a higher expected life span because they are more easily designed and have a higher aperture ratio. In top emission type organic EL devices, the cathode is generally formed on an organic EL layer. As a result, transmittance and optical efficiency of a top emission type organic EL devices are reduced because of a limited number of materials that may be used. If a thin film-type passivation layer is formed over the cathode to prevent a reduction of the light transmittance while protecting the cathode, the thin film-type passivation layer may fail to prevent infiltration of exterior air into the device.